Anthony Joshua could net £40million from his fight with Wladimir Klitschko

But the Olympic champ – who still lives with him beloved mum and does the chores around their ex-council flat – insists he will play it safe with his 10-figure fortune and only splash out on hard-earned holidays.

Joshua said: “They said I’m going to be a billionaire boxer? Is that before or after tax?

“What would I have to make before tax to be a billionaire boxer? Is that gross or net?

“What would I spend it on? Properties, then you have to leave some in the bank.

“Pensions, ISAs and bonds, that kind of stuff. And holidays, you’ve got to have holidays.”

Joshua’s promoter Eddie Hearn says his boxer’s humble lifestyle and out-of-ring appeal to the mainstream will help safeguard and grow his earnings.

Unlike Iron Mike, who frittered away millions, British hero Ricky Hatton, who has struggled with retirement, and David Haye - who according to upcoming opponent Tony Bellew is only making a comeback because he's skint - Joshua has a wise old head on giant shoulders and a determination to stay grounded.

Hearn said: "He's had and good year but a billion is a lot of money.

“He is earning phenomenal money both in and out the ring and this is the biggest fight in boxing history. It's also the biggest financially.